Filament cutting devices have generally taken the form of a power source connected or attached to a shaft which holds a whirling or rotating filament holding head or housing. Within the head or housing is at least one flexible filament that extends exteriorly of the housing. The head or housing is rotated at a speed sufficient to create line rigidity. The whirling filament is employed to cut grass or weeds and edge lawns. As a result of normal usage, the flexible filament often becomes worn, frayed or broken. The effective cutting length of the filament is thereby shortened. It is necessary, therefore, to provide within the filament cutting device an apparatus for uncoiling, paying out, or otherwise providing a new length of flexible filament. To avoid overloading the power source and also to conserve flexible filament, it is desirable to have an apparatus which permits the paying out of the filament to predetermined discrete increments only.
Apparatus are known in the prior art for metering a length of flexible filament. Many of the apparatus have metering devices which require the filament cutting devices to be bumped or pressed against the ground or other hard surface. The bumping disengages a first member from a second member thereby allowing a filament guide means to advance relative to a spool having filament thereon resulting in a lengthening of the swinging filament. The first and second members reengage after the axial displacement of the first and second members ceases. The bumping or pressing action, however, is not possible in many areas where filament cutting devices are advantageously useful, for example, on wheeled filament cutting devices or in gardens and other areas where the soil is soft or muddy.
Some filament cutting devices must be stopped in order to disengage first and second members, thereby releasing a filament spool to rotate relative to a filament guide means. With the spool free to rotate, the swinging portion of the filament may be lengthened by manually pulling it from the spool. These manually-operated devices obviously are not very convenient and tend to be quite time-consuming to operate.
Additionally, there are filament cutting devices which utilize the principle of rotational momentum during the deceleration or stopping of the cutting device to advance the spool portion of the device relative to the filament guide means portion. One such device is disclosed in a patent application by Gerald J. Zien entitled "Line Metering Apparatus," filed Feb. 12, 1979, U.S. Pat. No. 4,245,454 and assigned to the assignee of this invention. The line metering device disclosed therein includes a driving mechanism and a driven mechanism. A first set of teeth with ramp elements therebetween is formed on the top of the spool which is preferably coupled directly to the driving mechanism. A second set of teeth for engagement with the first set of teeth is formed on an undersurface of a guide drum which is coupled to the driven means and disposed in an overlying relationship with the filament spool. A bias means forces the second set of teeth on the guide drum into driving engagement with the first set of teeth on the filament spool. A means for rapidly decelerating the driving means is provided. When the driving means is rapidly braked with the decelerating means, the rotational momentum of the guide drum overcomes the bias force holding the first and second sets of teeth in an engaging relationship and allows the second set of teeth to slide up the ramp means thereby rotatingly moving the guide drum relative to the filament spool. The relative rotation of the guide drum and the filament spool results in a lengthening of the swinging portion of the filament. Thus, whenever an additional length of filament is required, the decelerating means is operated to allow the guide drum to rotate relative to the filament spool.
Another device utilizing rotational momentum is disclosed in U.S. Pat. No. 4,138,810. The metering device therein utilizes a flywheel engaged by gear means with filament spools. During deceleration of the filament cutting device, the rotational momentum of the flywheel causes it to overrun the filament spools and drive them by the gear means to rotate relative to the filament guide.
The present invention provides new and different means for metering a length of flexible filament utilizing the rotational momentum of one portion of a mechanism relative to another.